(Many thought that the pod person was going to be Superboy before Season 2 began and revealed Mon-El.)
The introduction of the Legion of Super-Heroes would be a major moment for television in general — not just the Arrow-verse.
In addition to Mon-El, the ever-evolving team has also featured heavy-hitters from the DCU such as Supergirl (obviously), Brainiac-5, Star Boy, Invisible Kid, Wildfire and Superboy.
Their adventures veer heavily into the realm of hard sci-fi, which makes Supergirl a perfect jumping off point for their stories.